The present invention relates to an endoprosthesis and to a method of producing such an endoprosthesis.
Hip endoprostheses are known which are composed of a prosthesis head, a prosthesis collar and a prosthesis shaft which is anchored with the use of bone cement or without cement in the medullary space of the femur of a patient. If the prosthesis shaft is anchored in the medullary space without cement, a shaft structure is employed which ensures the firmest possible anchorage in the medullary space, with a porous surface being provided to facilitate the in-growth of spongy material to thereby increase the strength of the anchorage of the prosthesis shaft.
However, the anchorage of the prosthesis shaft in the medullary space of the femur requires substantial removal of spongiosa from the intertrochantic region. Yet, the removal of spongiosa must be considered as a non-physiological local interference in the sense of defect formation since it adversely affects stress adaptation capabilities as well as primarily age-specific adaptation capabilities.
A further drawback of the prior art endoprosthesis is the irregular force distribution due to a primarily distal introduction of the forces into part of the thigh which is distal to the intertrochantic region, thus taking the bone portion intended for this purpose out of the functional load. This results in atrophy rather than in the desirable functional stimulation of the bone to cause it to grow onto the endoprosthesis in a fixing manner.
The consequence of both these drawbacks is premature loosening of the endoprosthesis and the resulting necessity of revisionary or corrective surgery. However, considerable difficulties are encountered in such revisionary or corrective surgery in connection with many types of endoprostheses because many of the respective prosthesis shafts are configured in such a way that removal of the endoprosthesis is possible only with considerable destruction of the bone material, which ultimately makes the growing-in of a new endoprosthesis more difficult and sometimes impossible.
A further drawback of prior art endoprosthesis is that standard models exist for different femur sizes, but these standard models are not adapted to the individual shape of the proximal femur of a patient. This causes any unfavorable force introduction in many endoprosthesis to be augmented, and therefore the existing bone substance is not utilized in an optimum manner.
Finally, the prior art endoprostheses require costly surgical techniques in which it is necessary to preliminarily chisel or drill the bone which is to receive the prosthesis so as to enable it to receive the prosthesis shaft. This must be done with the utmost of caution since it is necessary to avoid unnecessary damage to the bone structure as well as to avoid making a hole that is so large that the prosthesis shaft cannot be anchored with sufficient strength. Moreover, a considerable amount of time is required to rasp or drill open the bone, which considerably increases the risk of infection. Also, in these prior art devices it is necessary to open the medullary channel, thereby raising a considerable danger of bone destruction due to a via falsa, i.e. an incorrect channel opening which must be re-drilled.
A tibial plateau prosthesis is disclosed in German Offenlegungschrift (German unexamined patent application) No. 3,429,157 as a tibial plateau implant that is implanted without cement and is composed of a metal plate and a plastic slide bearing fastened thereon. It includes an insertion plate which is fastened to the metal plate on a side which is facing away from the slide bearing, the insertion plate being disposed essentially perpendicularly to the metal plate. In the prior art prosthesis, the insertion plate serves essentially to prevent tilting and/or displacement of the tibial plateau in spite of the cementless implantation when the knee joint is in a stressed bent position. To produce a firm connection between a tibial plateau and a tibia head, a bone screw is screwed through the insertion plate into the head of the tibia which, however, results in a more difficult attachment of the tibial plateau and, moreover, introduces additional stress on the tibia.
It is a problem in the art to provide a tibial plateau prosthesis which can be easily attached and connected with the tibia in a manner secure against displacement as well as tilting.